METAL FORMING 2.1 OVERVIEW OF METAL FORMING. Definitions CHAPTER CONTENTS

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1 2 METAL FORMING CHAPTER CONTENTS 2.1 Overview o Metal Forming Deinitions Material Considerations Temperature in Metal Forming Friction Eects 2.2 Bulk deormation Processes Classiication o Bulk Deormation Processes Rolling Forging Extrusion 2.3 Sheet Metalworking Classiication o Sheet Metalworking Processes Cutting Operations Bending Operations Deep Drawing Other Sheet Metalworking Operations High-energy-rate Forming (HERF) 2.1 OVERVIEW OF METAL FORMING Deinitions Plastic Deormation Processes Operations that induce shape changes on the workpiece by plastic deormation under s applied by various tools and dies. Bulk Deormation Processes These processes involve large amount o plastic deormation. The cross-section o workpiece changes without volume change. The ratio cross-section area/volume is small. For most operations, hot or warm working conditions are preerred although some operations are carried out at room temperature. Sheet-Forming Processes In sheet metalworking operations, the cross-section o workpiece does not change the material is only subjected to shape changes. The ratio cross-section area/volume is very high. Sheet metalworking operations are perormed on thin (less than 6 mm) sheets, strips or coils o metal by means o a set o tools called punch and die on machine tools called stamping presses. They are always perormed as cold working operations. Material considerations Material Behavior In the plastic region, the metal behavior is expressed by the low curve: σ = Κε n where K is the strength coeicient and n is the strain-hardening (or work-hardening) exponent. K and n are given in the tables o material properties or are calculated rom the material testing curves. Flow stress For some metalworking calculations, the low stress Y o the work material (the instantaneous value o stress required to continue deorming the metal) must be known: Y = Κε n 13

2 14 Metal Forming Valery Marinov, Manuacturing Technology Average (mean) low stress In some cases, analysis is based not on the instantaneous low stress, but on an average value over the strain-stress curve rom the beginning o strain to the inal (maximum) value that occurs during deormation: Y =K n Y Y Speciic energy u Stress-strain curve indicating location o average low stress Y in relation to yield strength Y and inal low stress Y The mean low stress is deined as Y n K 1 n here ε is the maximum strain value during deormation. Work-hardening It is an important material characteristic since it determines both the properties o the workpiece and process power. It could be removed by annealing. Temperature in metal orming The low curve is valid or an ambient work temperature. For any material, K and n depend on temperature, and thereore material properties are changed with the work temperature: log K n Increase in the work temperature log True stress-strain curve showing decrease in strength coeicient K and strain-hardening exponent n with work temperature There are three temperature ranges-cold, warm, and hot working: Cold working Warm working Hot working T A 0.3T m 0.5T m 0.75T m T m Temperature range or dierent metal orming operations. T A is the ambient (room) temperature, and T m is the work metal melting temperature

3 Valery Marinov, Manuacturing Technology Metal Forming 15 Cold working is metal orming perormed at room temperature. Advantages: better accuracy, better surace inish, high strength and hardness o the part, no heating is required. Disadvantages: higher s and power, limitations to the amount o orming, additional annealing or some material is required, and some material are not capable o cold working. Warm working is metal orming at temperatures above the room temperature but bel-low the recrystallization one. Advantages: lower s and power, more complex part shapes, no annealing is required. Disadvantages: some investment in urnaces is needed. Hot working involves deormation o preheated material at temperatures above the re-crystallization temperature. Advantages: big amount o orming is possible, lower s and power are required, orming o materials with low ductility, no work hardening and thereore, no additional annealing is required. Disadvantages: lower accuracy and surace inish, higher production cost, and shorter tool lie. Friction eects Homogeneous Deormation I a solid cylindrical workpiece is placed between two lat platens and an applied load P is increased until the stress reaches the low stress o the material then its height will be reduced rom initial value o h o to h 1. Under ideal homogeneous condition in absence o riction between platens and work, any height reduction causes a uniorm in-crease in diameter and area rom original area o A o to inal area A. h o A o work piece h A d o d Homogeneous deormation The load required, i.e. the press capacity, is deined by P = Y A Inhomogeneous deormation In practice, the riction between platens and workpiece cannot be avoided and the latter develops a barrel shape. This is called inhomogeneous deormation and changes the load estimation as ollows P d 1 h Y ks A Y 3 o o A

4 16 Metal Forming Valery Marinov, Manuacturing Technology h o work piece h riction s d o d Inhomogeneous deormation with barreling o the workpiece where µ is the rictional coeicient between workpiece and platen, and k s is the shape actor. 2.2 BULK DEFORMATION PROCESSES Classiication o Bulk Deormation Processes Basic bulk deormation processes (a) rolling, (b) orging, (c) extrusion, (d) drawing Rolling: Forging: Extrusion: Drawing: Compressive deormation process in which the thickness o a plate is reduced by squeezing it through two rotating cylindrical rolls. The workpiece is compressed between two opposing dies so that the die shapes are imparted to the work. The work material is d to low through a die opening taking its shape The diameter o a wire or bar is reduced by pulling it through a die opening (bar drawing) or a series o die openings (wire drawing)

5 Valery Marinov, Manuacturing Technology Rolling Metal Forming 17 Deinition Rolling is a Bulk Deormation Process in which the thickness o the work is reduced by compressive s exerted by two opposing rolls: The process o lat rolling Steps in rolling The preheated at 1200 o C cast ingot (the process is known as soaking) is rolled into one o the three intermediate shapes called blooms, slabs, or billets. v v v Bloom has a square cross section o 150/150 mm or more Slab (40/250 mm or more) is rolled rom an ingot or a bloom Billet (40/40 mm or more) is rolled rom a bloom These intermediate shapes are then rolled into dierent products as illustrated in the igure: Production steps in rolling

6 18 Metal Forming Valery Marinov, Manuacturing Technology Next pictures show some production steps in lat and shape rolling: Powerul tongs lit an ingot rom the soaking pit where it was thoroughly heated to the rolling temperature Steel bloom enters the rolling mill Structural shapes are rolled rom blooms on mills equipped with grooved rolls Hot saw cuts rolled shapes to customer length ater delivery rom the inishing rolling mill

7 Valery Marinov, Manuacturing Technology Flat rolling Metal Forming 19 Work velocity V Roll velocity V r V o Neutral point L Side view o lat rolling and the velocity diagram indicating work and roll velocities along the contact length L The work is squeezed between two rolls so that it thickness is reduced by an amount called the drat, d d = t o -t I the drat is expressed as a raction o the starting block thickness, it is called reduction, r: r = d/t o Rolling increases the work width rom an initial value o w o to a inal one o w, and this is called spreading. The inlet and outlet volume rates o material low must be the same, that is, t o w o v o = t w v where v o and v are the entering and exiting velocities o the work. The point where roll velocity equals work velocity is known as the no-slip point or the neutral point. The true strain and the mean low stress are deined by to true strain ln, and mean low stress t Y n K 1 n Friction occurs with a certain coeicient o riction µ on either sides o no-slip point. Both riction s act in opposite directions and are not equal. The entrance is bigger so that the resulting pulls the work through the rolls. The maximum possible drat d max depends on µ and roll radius R and is given by The rolling F is estimated as where L is the contact length, approximately d max = µ 2 R F Y wl L R( t o t The power P required to drive each roll is where N is the rotational speed o the roll. P=2πNFL

8 20 Metal Forming Valery Marinov, Manuacturing Technology Shape rolling The work is deormed by a gradual reduction into a contoured cross section (I-beams, L-beams, U-channels, rails, round, squire bars and rods, etc.). Ring rolling Thick-walled ring o small diameter is rolled into a thin-walled ring o larger diameter: Ring rolling used to reduce the wall thickness and increase the diameter o a ring Thread rolling Threads are ormed on cylindrical parts by rolling them between two thread dies: Gear rolling Thread rolling with lat dies Gear rolling is similar to thread rolling with three gears (tools) that orm the gear proile on the work. Work Gear rolls Gear rolling between three gear roll tools

9 Valery Marinov, Manuacturing Technology Metal Forming 21 Forging Deinition Forging is a Bulk Deormation Process in which the work is compressed between two dies. According to the degree to which the low o the metal is constrained by the dies there are three types o orging: Open-die orging Impression-die orging Flashless orging Three types o orging: (a) open-die orging, (b) impression die orging, and (c) lashless orging Open-die orging Known as upsetting, it involves compression o a work between two lat dies, or platens. Force calculations were discussed earlier. Sequence in open-die orging illustrating the unrestrained low o material. Note the barrel shape that orms due to riction and inhomogeneous deormation in the work Open-die orging o a multi diameter shat

10 22 Metal Forming Valery Marinov, Manuacturing Technology Impression-die orging In impression-die orging, some o the material lows radially outward to orm a lash: Schematics o the impression-die orging process showing partial die illing at the beginning o lash ormation in the center sketch, and the inal shape with lash in the right-hand sketch Stages (rom bottom to top) in the ormation o a crankshat by hot impression-die orging Estimation o the maximum F can be approximately done by F = K Y A where K is the shape actor ranging rom 6 to 10, bigger or more complex shapes, Y is the yield strength o the material at work temperature, A is the projected area o the part, including lash. Flashless orging The work material is completely surrounded by the die cavity during compression and no lash is ormed: Flashless orging: (1) just beore initial contact with the workpiece, (2) partial compression, and (3) inal push and die closure. Symbol v indicates motion, and F - applied. Most important requirement in lashless orging is that the work volume must equal the space in the die cavity to a very close tolerance. For estimation, the same equation as in impression-die orging is applied.

11 Valery Marinov, Manuacturing Technology Metal Forming 23 Coining Special application o lashless orging in which ine detail in the die are impressed into the top and bottom suraces o the workpiece. There is a little low o metal in coining. Coining operation: (1) start o cycle, (2) compression stroke, and (3) ejection o inished part Forging machines The next igures show some examples o the common orging machines-hammers and presses: Drop orging hammer, ed by conveyor and heating unit at the right o the scene. A ton orging press. In the oreground is a 120-kg, 3-m aluminum part that has orged on this press.

12 24 Metal Forming Valery Marinov, Manuacturing Technology Extrusion Deinition Extrusion is a Bulk Deormation Process in which the work is d to low through a die opening to produce a desired cross-sectional shape. Typical shapes produced by extrusion Extrusion is perormed in dierent ways thereore dierent classiications are available: v v v Direct and indirect extrusion Hot and cold extrusion Continuous and discrete extrusion Direct and indirect extrusion (Let) Direct extrusion to produce hollow or semihollow cross section. (Right) Direct extrusion to produce solid cross section. Schematic shows the various equipment components. Force and power analysis in extrusion The ram, F, is estimated as F = p A o where A o is the billet cross-sectional area, and p is the ram pressure, 2L p Y x Do where D o is the original diameter o the billet, L is the length o the billet in the die, and ε x is the extrusion strain, ε x = a+b ln(a o /A ), a and b being the empirical constants, usually a=0.8 and b=1.2~1.5. Power required is calculated as P = Fv, where v is the ram velocity. In indirect extrusion (backward, inverse extrusion) the material lows in the direction opposite to the mo-tion o the ram to produce a solid (top) or a hollow cross section (bottom).

13 Valery Marinov, Manuacturing Technology Metal Forming 25 Wire and Bar Drawing Deinition Wire and Bar Drawing is a Bulk Deormation Process in which the cross-section o a bar, rod or wire is reduced by pulling it through a die opening, as in the next igure: Drawing o a rod, bar, or wire Bar drawing is a single-drat operation. By contrast, in wire drawing the wire is drawn through a series o dies, between 4 and 12. The drat, d, is deined as and reduction, r, is given by d = D o - D r = d/d o Force and power analysis in drawing The draw F is calculated as a product o the drawn cross-section area A and the draw stress σ d F = A σ d The draw stress σ d is deined as d Y 1 tan A ln A where φ is the actor, that accounts or inhomogeneous deormation, usually around 1.0. o